Azithromycin is a well-known semi-synthetic macrolide antibiotic. It is prepared through the ring expansion to incorporate a nitrogen atom in the macrolide ring of erythromycin A, followed by reductive methylation. This provides an antibiotic having more stability and greater effectiveness than erythromycin-A.
The ring expansion and subsequent conversion of erythromycin-A to provide azithromycin is described in U.S. Pat. No. 4,474,768, (e.g., Example 3). Generally, the synthesis requires several steps. The product obtained is one of the hydrated versions, either monohydrate or dihydrate.
Azithromycin monohydrate is hygroscopic and thus, difficult to maintain in the monohydrated form. U.S. Pat. No. 4,963,531 and EP application 298 650 teach a process for preparing azithromycin dihydrate. The process requires preparing a solution of azithromycin monohydrate in tetrahydrofuran and water. The azithromycin dihydrate is obtained by crystallization upon addition of hexane.
In U.S. Pat. No. 4,963,531 it is disclosed that on storage at low humidity the azithromycin dihydrate loses water. In addition, samples of azithromycin mono- and di-hydrate stored at higher humidity rapidly absorbed water. Thus, the water percentage (percent hydration) in the crystals can vary depending on the relative humidity during storage. This variability of the percent hydration can make it difficult to accurately determine the proper amount of active ingredient needed to prepare various dosage forms.
Thus, there is a need for forms of azithromycin that exhibit stability and less variability in the level of hydration.